what is abc?
abc is a simple text based music notation system. It can be read and written without the aid of a computer, or can be run through various pieces of software to produce standard music notation, MIDI files, or to playback.abc packages for the Macintosh
[there are more, but see here for Five Line Skink ]Find out more
The abc notation is described at Chris Walshaw's abc page.Steve Mansfield has an excellent tutorial and FAQ.
John Chambers has a large set of abc resources at http://trillian.mit.edu/~jc/music/abc/, including his abc Tune Finder and a FAQ, as well as a primer at http://trillian.mit.edu/~jc/music/abc/doc/ABCprimer.html